This invention relates to an apparatus that identifies a reversed or missing integrated circuit (IC) lead frame by means of an electrical circuit that monitors pin contact or non-contact with the lead frame as it sets in a hot plate chase, in preparation for an encapsulation process.
The process of encapsulation of a conventional IC die comprises the following steps:
1. the dies are formed;
2. the dies are attached to a metal lead frame having a (typically, but not exclusively) oblong hole on one rail and circular "pin one" holes on the opposite rail;
3. the lead frames are guided onto a loading frame by the "hot plate chase" used to heat the lead frames;
4. the lead frames are visually inspected by an operator to verify proper orientation in the hot plate chase, i.e. the pin one holes are oriented correctly with respect to the encapsulation mold;
5. the loading frame carrier top is closed over the hot plate chase and lead frames; and
6. the loading frame carrier is transferred to the encapsulation mold for a pressurized, resin injection, encapsulation process.
The result of a reversed lead frame is that the IC dies on the lead frame will be rotated 180.degree. from the proper orientation. For instance, the pin 9 lead will be incorrectly marked as pin 1 (on a 16 pin dual in-line package (DIP)). Additionally, a reversed lead frame will have the oblong aperture on the side where the pin 1 holes should be, causing poor alignment during encapsulation and later in the process when the lead frames are punched and the plastic package is trimmed. Misalignment in turn causes improper trimming, improper forming and most significantly, possible damage to the trimming and forming machines.
A missing lead frame will cause a complete cycle of the encapsulation process to be defective for as many as 200 or more dies, since each hot plate chase carries two lead frames, each having a plurality of dies, with as many as six to ten hot plate chase carriers mounted together on one hot plate. Defective encapsulation occurs due to low pressure in the mold since it is pumping plastic into the void that should have contained a lead frame. Not only are up to 200 or more dies rejected but the spilled plastic creates a significant maintenance problem. As noted above, the current method of monitoring for reversed dies is a visual inspection of the lead frames in the hot plate chase to verify that the lead frames have the proper pin one orientation on the hot plate. This is not a fool-proof task due to human error: the pin one holes are about 0.062 inches in diameter and the oblong apertures are typically oval-shaped with a small dimension of 0.062 inches and a long dimension of about 0.080 inches. Mistakes in visual observation are easily made.
It is the purpose of this invention to provide an apparatus that automatically monitors the hot plate chase to verify that the lead frame orientation is correct, and if a lead frame is reversed, the apparatus will provide appropriate indication or alarm means to warn the operator. A second embodiment of the invention will also detect a missing lead frame from the hot plate chase and provide appropriate indication or alarm.